IIS[Vista+IIS7+APS.Net]无法发送邮件

以下code在一个c# console程序中执行就可以发送邮件成功，但是作为一个web service通过web去调用，就说client has not right to connect to email server.!

MailMessage mailMsg = new MailMsg();!
mailMsg.to ...!
mailMsg.cc ...!
mailMsg.subject ...!


SmtpClient client = new SmtpClient("smtp.server.com");!
client.Credentials = CredentialCache.DefaultNetWorkCredential;!

client.Send (mailMsg);!